1. Field of the Invention
This invention relates to the detection of electromagnetic radiation with a multiple quantum well (MQW) superlattice structure, and more particularly to the sensing of long wavelength infrared radiation (LWIR) with an MQW superlattice that is subject to a significant dark current level.
2. Description of the Related Art
MQW superlattice LWIR detectors made of heterojunction materials, such as GaAs/Ga.sub.x Al.sub.1-x As, provide excellent design flexibility for spectral response. Because of the large bandgap of the semiconductive materials employed and the ultra-thin (typically less than 1 micron) structure required, the detector offers potentially superior radiation hard characteristics as compared with narrow bandgap HgCdTe-based or thicker (about 20 microns) extrinsic silicon devices. The detection of LWIR with an MQW sensor has been reported in several publications, such as Levine et al., "Bound-to-Extended State Absorption GaAs Superlattice Transport Infrared Detectors", J. Applied Physics Letters, Vol. 64, No. 3, 1 Aug. 1988, pages 1591-1593; Levine et al., "Broadband 8-12 .mu.m High-Sensitivity GaAs Quantum Well Infrared Photodetector", Applied Physics Letters, Vol, 54, No. 26, 26 Jun. 1989, pages 2704-2706; Hasnain et al., "GaAs/AlGaAs Multiquantum Well Infrared Detector Arrays Using Etched Gratings", Applied Physics Letters, Vol. 54, No. 25, 19 Jun. 1989, pages 2515-2517; Levine et al., "High-Detectivity D*=1.0.times.10.sup.10 cm.sqroot.Hz/W GaAs/AlGaAs Multiquantum Well .lambda.=8.3 .mu.m Infrared Detector", Applied Physics Letters, Vol. 53, No. 4, 25 Jul. 1988,, pages 296-298.
The principal of operation for an MQW superlattice IR detector is illustrated in FIG. 1. The basic device consists of a periodic heterostructure of GaAs quantum wells 2 and AlGaAs barrier layers 4. The GaAs quantum well layers are doped with an n-type dopant, such as silicon, to provide electrons in the ground states of the wells for intersubband detection.
The thickness of each quantum well layer 2 is sufficiently small, preferably about 20-60 Angstroms and most preferably about 40 Angstroms, that quantum effects are significant. The thickness of each barrier layer 4 is generally about 80-300 Angstroms, and most preferably about 140 Angstroms. The superlattice period is thus preferably about 180 Angstroms. To maximize the device's quantum efficiency, it is desirable to provide many quantum well layers 2. Photon absorption occurs in the quantum well layers, and quantum efficiency is thus a function of the number of such layers. Although a smaller number of periods is shown in FIG. 1, it is generally preferred that the superlattice comprise about a 20-30 period structure. GaAs quantum well layers 2 are heavily doped n-type with a donor impurity such as Ge, S, Si, Sn, Te or Se. A particularly preferred dopant is Si at a concentration of about 1.times.10.sup.18 -5.times.10.sup.18 cm.sup.-3, and most preferably about 5.times.10.sup.18 cm.sup.-3. Lattice match and thermal coefficient considerations, impurity concentrations and fabrication techniques are known in the art.
Although a GaAs/AlGaAs superlattice is preferred, other materials may also be used. For example, it may be desirable to use materials such as InGaAs/InAlAs on InP, SiGe on Si, or HgCdTe. In general, superlattices fabricated from III-V, IV-IV and II-VI semiconductor materials are suitable. The MQW superlattice detectors are particularly suited for the detection of LWIR, but the sensors in general are applicable to the detection of radiation and other wavelength regimes, and no limitation to LWIR for the present invention is intended.
The potential energy barrier height E.sub.b of the barrier layers 4 is about 160 meV above the potential energy barrier height E.sub.w of the quantum wells 2 for GaAs/AlGaAs. For LWIR with peak detection of about 12 microns, the energy gap between the bound state and the excited state for electrons in the quantum wells is about 100 meV, with the first electron excited state in the quantum wells lying above the conduction band edge of the barrier layers.
Incident infrared photons excite electrons from the quantized baseband 6 of the wells to extended excited states in continuous conduction subband 8, which has an energy level greater than the conduction band floor for barrier layers 4. These excited electrons are then accelerated towards a collector by an electric field created by an externally applied bias voltage. Under normal sensor operating conditions, the bias voltage causes the mean-free path of electrons in the subband 8 to be sufficiently large for the electrons to travel under the applied field through the superlattice, producing a photocurrent 10 that is measured as an indication of the magnitude of incident radiation.
The sensitivity of an MQW superlattice infrared detector can be severely limited by high levels of dark current. This current consists primarily of electrons which tunnel through the intervening barrier layers 4 between the ground states of adjacent quantum wells 2, and is indicated by arrows 12. The tunneling current can be reduced by in creasing the widths of the barrier layers 4. However, any such increase in the barrier layer width reduces the device's radiation hardness, which is inversely related to its thickness.
An improvement upon the described detector, illustrated in FIG. 2, is disclosed in pending U.S. patent application Ser. No. 07/457,613, filed Dec. 27, 1989 by Sato et al., "Dark Current-Free Multiquantum Well Superlattice Infrared Detector", and assigned to Hughes Aircraft Company, the assignee of the present invention. Under this approach the barrier layers 4 are kept thin, but a thicker (generally about 800-3,000 Angstroms) tunneling current blocking layer 14 is provided at the end of the superlattice in the path of the tunneling electrons. An electron emitter contact layer 16 is formed at the far end of the superlattice from the blocking layer, while an electron collector layer 18 is formed on the opposite side of the blocking layer from the superlattice. The blocking layer 14, which is preferably formed from the same material as the barrier layers 4, eliminates most of the tunneling current component of the photodetector's dark current. This in turn allows the individual barrier layers 4 to be made thinner, thus enhancing the detector's quantum efficiency and increasing its radiation hardness.
While the addition of the blocking layer 14 produces an extremely low dark current and desirable photo-voltage operating characteristics, the device as thus modified is not readily adaptable to receiving an external bias voltage. This is because the blocking layer 14 is a high resistance region compared to the MQW superlattice, and most of the bias voltage drop is diverted from the superlattice to the blocking layer. The performance improvement that otherwise would be available by applying a bias voltage across the superlattice is thus prevented.